Accelerometers are found in many modern electronic devices, and are used in order to determine acceleration. Microelectromechanical Systems (MEMS)-based accelerometers have become ubiquitous in recent years, and are often far more effective than their conventional macroscopic counterparts.
It is known in the art per se to use interdigitated capacitive electrode “fingers” to sense acceleration. An example of such a capacitive accelerometer is described in U.S. Pat. No. 7,047,808. When the accelerometer undergoes acceleration, a moveable set of capacitive electrode fingers attached to a proof mass tends to move from a null position relative to a fixed set of capacitive electrode fingers attached to an inertial frame.
Capacitive accelerometers can be operated either in what is known as “open loop” or “closed loop”. In an open loop system, the capacitive accelerometer is arranged to sense the change in capacitance between the electrode fingers caused by the relative movement of the proof mass e.g. a so-called “pick off” voltage is used to determine acceleration. In a closed loop system, the electrode fingers are used for both driving and sensing. Typically, in-phase and anti-phase pulse width modulation (PWM) voltage waveforms are applied to the fixed electrode fingers and the PWM mark/space ratio is adjusted to drive the proof mass back to its null position (i.e. the position it would be in were it not undergoing acceleration). The mark/space ratio provides a linear measure of acceleration.
Capacitive accelerometers, especially those operated in closed loop, require a very low vibration rectification error (VRE). As the response of a capacitive accelerometer is non-linear with respect to motion, due to the non-linear dependence of electrostatic force with proof mass position (the electrostatic force is inversely proportional to the square of the gap between electrodes), vibration causes the accelerometer to have a DC acceleration output even though it is not accelerating, resulting in a significant VRE. Reducing VRE can be achieved by reducing the residual proof mass motion. In closed loop, the residual motion can be reduced by having a high gain around the loop which normally requires a high bandwidth. However, the maximum gain that can be applied is limited, and the bandwidth is also limited by virtue of computational delays and the resonance frequency of the MEMS, so some degree of residual motion is inevitable. Any delay in the loop limits the bandwidth and thus the maximum open loop gain.
Furthermore, in the case of both open and closed loop systems, it is also important to ensure that the moveable set of electrode fingers does not move too much if a capacitive accelerometer undergoes large “shock” accelerations. If the moveable set of fingers moves too far, it will “touch down” i.e. it will make physical contact with the fixed set of fingers. While there are bump stops put in place in order to prevent this happening, at high levels of acceleration it can still occur due to finger flexure and this limits the operational range of the accelerometer. Shocks may cause damage to the accelerometer, particularly as the interdigitated sets of electrode fingers will be at different electrical potentials; For example, the Gemini range of MEMS accelerometers available from Silicon Sensing provides a dynamic range up to ±96 g with an operating shock of 1000 g.
As there are relatively small spaces between the electrode fingers, an effect known as squeeze film damping takes place, wherein a gaseous medium between the electrode fingers damps the motion of the fingers due to the viscosity of the gaseous medium. The residual motion of the proof mass is therefore damped to a degree by this squeeze film damping effect. However, there are inherent limits to the geometry of the interdigitated electrode fingers, particularly for closed loop accelerometers wherein the fingers have to be stiff enough in order to prevent them deforming under an applied voltage (e.g. 35 V) and this requires a minimum thickness. Typically, the capacitive electrode fingers are trapezoidal with a typical width of 20 microns at the root, tapered to a width of around 12 microns at the tip. Also there needs to be a sufficient gap either side of each finger, wherein there is an offset between successive adjacent capacitive electrode finger pairings, such that there is a larger gap on one side of any given electrode finger than on the other side. Typical spacing between the interdigitated capacitive electrode fingers may be around 6 microns for the smaller gap and 16 microns for the larger gap. The resulting pitch of the sets of interdigitated capacitive electrode fingers (i.e. the spacing between pairs of fingers within the set) is typically around 50 microns, which limits the effectiveness of the squeeze film damping in counteracting residual motion.
It would be desirable to reduce the residual motion of the proof mass in a capacitive accelerometer to provide various benefits. The present disclosure seeks to reduce or overcome the disadvantages outlined above.